Not applicable.
Not applicable.
1. Field of the Invention
The present invention generally relates to a logging-while-drilling (LWD) tool that measures the resistivity of formations adjacent the wellbore. More particularly, the present invention relates to an LWD focused resistivity tool with multiple transmitters to provide multiple depths of investigation. Still more particularly, the present invention relates to a bottomhole drilling assembly that includes an LWD resistivity tool for determining characteristics of the borehole and formation during the drilling of a well, and correlating that information with depth to produce an image of some desired portion of the borehole.
2. Background of the Invention
Modem petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as xe2x80x9clogging,xe2x80x9d can be performed by several methods. Logging has been known in the industry for many years as a technique for providing information regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or xe2x80x9csondexe2x80x9d is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed steel cylinder for housing the sensors, which hangs at the end of a long cable or xe2x80x9cwireline.xe2x80x9d The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface, and control signals from the surface to the sonde. In accordance with conventional techniques, various parameters of the earth""s formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
The sensors used in a wireline sonde may include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensors have been used to determine particular characteristics of the formation, including resistivity sensors, nuclear sensors, and acoustic sensors. See generally J. Labo, A Practical Introduction to Borehole Geophysics (Society of Exploration Geophysicists 1986); xe2x80x9cSix Arm Dipmeter,xe2x80x9d Halliburton Logging Services (copyright 1989).
While wireline logging is useful in assimilating information relating to formations downhole, it nonetheless has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drillstring and bottomhole assembly first must be removed or xe2x80x9ctrippedxe2x80x9d from the borehole, resulting in considerable cost and loss of drilling time for the driller (who typically is paying daily fees for the rental of drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, the drilling service company must at times make decisions (such as the direction to drill) possibly without sufficient information, or else incur the cost of tripping the drillstring to run a logging tool to gather more information relating to conditions downhole. In addition, because wireline logging occurs a relatively long period after the wellbore is drilled, the accuracy of the wireline measurement can be compromised. As one skilled in the art will understand, the wellbore conditions tend to degrade as drilling fluids invade the formation in the vicinity of the wellbore. Consequently, a resistivity tool run one or more days after a borehole section has been drilled may produce measurements that are influenced by the resistivity of the mud that has invaded the formation. In addition, the shape of the borehole may begin to degrade, reducing the accuracy of the measurements. Thus, generally, the sooner the formation conditions can be measured, the more accurate the reading is likely to be. Moreover, in certain wells, such as horizontal wells, wireline tools cannot be run.
Because of these limitations associated with wireline logging, there is an increasing emphasis on developing tools that can collect data during the drilling process itself. By collecting and processing data and transmitting it to the surface real-time (or near real-time) while drilling the well, the driller can more accurately analyze the surrounding formation, and also can make modifications or corrections, as necessary, to optimize drilling performance. With a steerable system the driller may change the direction in which the drill bit is headed. By detecting the adjacent bed boundaries, adjustments can be made to keep the drill bit in an oil bearing layer or region. Moreover, the measurement of formation parameters during drilling, and hopefully before invasion of the formation, increases the usefulness of the measured data. Further, making formation and borehole measurements during drilling can save the additional rig time which otherwise would be required to run a wireline logging tool.
Designs for measuring conditions downhole and the movement and the location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as xe2x80x9cmeasurement-while-drillingxe2x80x9d techniques, or xe2x80x9cMWD.xe2x80x9d Similar techniques, concentrating more on the measurement of formation parameters of the type associated with wireline tools, commonly have been referred to as xe2x80x9clogging while drillingxe2x80x9d techniques, or xe2x80x9cLWD.xe2x80x9d While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used generically with the understanding that the term encompasses systems that collect formation parameter information either alone or in combination with the collection of information relating to the position of the drilling assembly.
Ordinarily, a well is drilled vertically for at least a portion of its final depth. The layers or strata that make up the earth""s crust are generally substantially horizontal. Therefore, during vertical drilling, the well is substantially perpendicular to the geological formations through which it passes. In certain applications, however, such as when drilling from an off-shore platform, or when drilling through formations in which the reservoir boundaries extend horizontally, it is desirable to drill wells that are oriented more horizontally. When drilling horizontally, it is desirable to maintain the well bore in the pay zone (the formation which contains hydrocarbons) as much as possible so as to maximize the recovery. This can be difficult since formations may dip or diverge. Thus, while attempting to drill and maintain the well bore within a particular formation, the drill bit may approach a bed boundary. Many in the industry have noted the desirability of an LWD system that could be especially used to detect bed boundaries and to provide real-time data to the driller to enable the driller to make directional corrections to stay in the pay zone. Alternatively, the LWD system could be used as part of a xe2x80x9csmartxe2x80x9d system to automatically maintain the drill bit in the pay zone. See, e.g. commonly assigned U.S. Pat. No. 5,332,048, the teachings of which are incorporated by reference herein. The use of an LWD system with these other systems makes it possible to conduct at least certain portions of drilling automatically.
The measurement of formation properties during drilling of the well by LWD systems thus improves the timeliness of measurement data and, consequently, increases the efficiency of drilling operations. Typically, LWD measurements are used to provide information regarding the particular formation through which the borehole crosses. Currently, logging sensors or tools that commonly are used as part of either a wireline or an LWD system include resistivity tools. For a formation to contain hydrocarbons and permit the hydrocarbons to flow through it, the rock comprising the formation must have certain well known physical characteristics. One characteristic is that the formation has a certain measurable resistivity (the inverse of conductivity), which can be determined by appropriate transducers in the drill string. Analysis of the data from these transducers provide information regarding the resistivity of the formation surrounding the resistivity tool, which then can be used in combination with other measurements to predict whether the formation will produce hydrocarbons. In addition, a sudden measured change in resistivity at the boundary between beds of shale and sandstone can be used to locate these boundaries. In horizontal drilling, the drill bit preferably can then be steered to avoid this boundary and keep the wellbore inside the oil-producing bed. However, to accomplish this detection reliably, a great deal of data is required from the resistivity tool.
To prevent blowouts, wells typically are drilled with a positive hydrostatic pressure so that the pressure in the borehole is greater than the pressure in the formation. The positive hydrostatic pressure in the borehole results from pumping specially formulated drilling mud into the wellbore during the drilling process. Because the drilling mud is maintained at a higher pressure than the formation, the mud tends to invade the permeable formation surrounding the borehole, forcing the original connate water to be driven away from the borehole. This flushing of drilling mud filtrate into the formation creates an invaded or flushed zone around the borehole, with a transition zone between the flushed and undisturbed zones. The depth of invasion of the drilling mud is a factor of the formation porosity, the differential drilling pressure, permeability of the formation, water loss of the drilling fluid, and time.
Because of this invasion of the formation by the drilling fluid, it is generally desirable for the resistivity tool to measure at multiple depths into the formation around the borehole between the transmitter and receiver. By using several resistivity sensors, with each responding predominately to a different depth of investigation (such as deep, medium and shallow), the deeper reading sensors can be corrected based upon the measurements obtained from the shallower reading sensors.
Thus, referring to FIG. 1, the first and closest diameter of investigation relative to the resistivity tool is the area within the wellbore through which drilling mud flows back to the surface. If the resistivity of this area is measured inside the wellbore (around the tool itself), a resistivity value will be obtained that generally approximates the resistivity of the drilling mud, Rm. This diameter of investigation can be referred to as Dm, to denote that this is the depth of investigation that will produce a resistivity reading of the drilling mud. The next general area of investigation is the region within the surrounding formation that has been invaded by the drilling mud. This diameter of investigation can be referred to as Di, because a resistivity measurement in this region will yield the resistivity of the invaded zone, which may be denoted as Rxo. The third region of investigation for a resistivity tool is the formation which has not been invaded by drilling mud. A resistivity measurement of this region will yield the true resistivity value of the formation, Rt. While information regarding Rm and Rxo are useful for purposes of evaluation, one of the goals of the resistivity tool is to measure the true formation resistivity, Rt. Thus, it is important to design the resistivity tool to have sufficient depths of investigation to measure this resistivity.
As one skilled in the art will understand, there are various types of resistivity measuring tools used to log wellbores. As described generally in Darwin Ellis, Well Logging for Earth Scientists, pp. 84-91 (Elsevier 1987), focused resistivity or laterologs are electrode devices that force a measuring current into the formation. The concept of focusing is illustrated in FIG. 2, where three current emitting electrodes A0, A1, and A1xe2x80x2 are shown in a Laterolog-3 configuration. The potential of electrodes A1 and A1xe2x80x2 is held constant and at the same potential as the central electrode A0. Because current only flows if a potential difference exists between the electrodes, theoretically no current flows vertically between the electrodes. Thus, as shown in FIG. 2, a sheath of current emanates horizontally from the central electrode A0. The amount of current emanating from electrode A0 can be used to determine the resistivity of the formation using Ohm""s Law.
An implementation of a focused resistivity device is disclosed in U.S. Pat. No. 3,305,771, issued to Arps. As described in that patent, a pair of toroidal transmitters are mounted in a logging sonde, positioned above and below a pair of toroidal receivers. An alternating current generator excites the toroidal transmitters, which induces current into the formation. The receivers are symmetrically located with respect to the transmitters, and detect the current that passes out of the collar into the formation between the two receivers. Because the source voltage is known, resistivity of the formation in the vicinity of the receivers can be determined as:
R=k(V/I),
where R is the formation resistivity, V is the source voltage, I is the measured current flowing out into the formation between the toroidal receivers (i.e. the difference in current measured at each receiver), and k is a tool constant dependent on the spacing of the toroids.
Resistivity tools based generally upon the system disclosed in Aarps have been used for many years. One example of such a tool is found in S. Bonner, et al., xe2x80x9cA New Generation of Electrode Resistivity Measurements For Formation Evaluation While Drilling,xe2x80x9d SPWLA 35th Annual Logging Symposium, Jun. 19-22, 1994. See also U.S. Pat. No. 5,339,037. A simple illustration of the LWD tool described in the Bonner et al. article is depicted in FIG. 3. The LWD tool disclosed in this article makes five formation resistivity measurements using two toroidal transmitters. One resistivity measurement uses the drill bit as part of the measuring electrode. The other four resistivity measurements are characterized as high vertical resolution electrode resistivities that are focused. One of the high vertical resolution measurements uses a ring electrode to make an azimuthally averaged resistivity. The other three high vertical resolution electrodes use button electrodes that are vertically aligned to make azimuthally sensitive resistivity measurements. Together the ring and buttons give a total of four depths of investigation.
While the LWD tool disclosed in the Bonner et al. article provides multiple depths of investigation it has some serious drawbacks. One of those is that three of the four high vertical resolution resistivity measurements are azimuthally sensitive. Thus, three of the four high resolution measurements are sensitive to the orientation of the tool in the borehole. This can be problematic if the bottomhole assembly is not rotating. Thus, if the bottomhole assembly is being steered (or is xe2x80x9cslidingxe2x80x9d), the LWD tool in Bonner et al will have only one sensor that obtains high resolution resistivity measurements around the borehole. The other three sensors will point in the same direction, and thus will not be able to capture resistivity measurements around the entire circumference of the borehole. Similarly, if a drill string is used that is not rotated during normal drilling operations, the Bonner et al. system will have limited application. Thus, in applications where the drill string is not rotated, the Bonner et al. tool will not obtain an image of the borehole from the button electrodes.
Although the Bonner et al. design incorporates a two transmitter configuration, it is known to use additional transmitters to obtain more depths of investigation in resistivity measurements. For example, it has been suggested that four transmitters be used with a pair of receivers in a standard resistivity tool. See M.S. Bittar, et al., xe2x80x9cA True Multiple Depth of Investigation Electromagnetic Wave Resistivity Sensor: Theory, Experiment and Prototype Field Test Results,xe2x80x9d presented at the 66th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers on Oct. 6-9, 1991; S. Ball, et al., xe2x80x9cFormation Evaluation Utilizing a New MWD Multiple Depth of Investigation Resistivity Sensor,xe2x80x9d presented at the Fifteenth European Formation Evaluation Symposium on May 5-7, 1993. Each transmitter fires sequentially, with attenuation and phase shift measurements being made based on the amplitude and time of the signals received by the receiver pair.
Attempts have been made to develop LWD tools that may be used to provide an image of the borehole. Imaging tools have been used in wireline tools for a number of years to obtain snapshot images of the borehole at particular depths. Thus, for example, the assignee of the present invention has used an acoustic logging tool as a wireline imaging device. See Open Hole Services, (Halliburton Logging Services 1992), p. 28. This device is commonly referred to as the Circumferential Acoustic Scanning Tool (or CAST). An example of an LWD imaging tool is shown in commonly assigned U.S. Pat. No. 5,899,958.
While the above tools and systems work well in their intended applications, it would be desirable to develop an LWD resistivity tool that can obtain focused resistivity measurements at multiple depths, while also providing an image of the borehole regardless of whether the tool is rotating. While the advantages of such a tool are immediately apparent to one skilled in the art, to date no one has successfully implemented such a system which overcomes the limitations listed above.
The problems noted above are solved in large part by a focused resistivity logging tool that includes multiple toroidal transmitters positioned symmetrically or asymmetrically with respect to a toroidal receiver pair to obtain three different depths of investigation. In addition, a plurality of button or rectangular electrodes are positioned around the circumference of the logging tool to provide borehole imaging using the same transmitter and receiver array. The use of multiple button electrodes around the circumference of the tool enables imaging of the borehole, even if the drillstring is not rotating, as may occur when the bottomhole assembly is being steered, or when special drill string materials are used. In the preferred embodiment, the drill bit also is used to obtain a resistivity at the bit that can be used for early detection of bed boundaries.
In the preferred embodiment, each of the transmitters are alternatively energized to induce an axial current in the tool. The current flowing into the formation between the receiver coils is determined by measuring the axial current at each of the toroidal receivers. The difference in axial current measured by the receivers indicates the current flowing into the formation. This current measurement can then be used to determine the resistivity of the formation using Ohm""s Law. The multiple transmitters enable current measurements from differently spaced transmitters, thus providing multiple depths of investigation.
In one embodiment of the invention, one or more ring electrodes are provided in the vicinity of the toroidal receivers and are used to measure formation resistivity . In addition to the measurement of axial current at the toroidal receivers, which determine the radial current Is, the voltage (Vring) at the ring electrode is measured and used to determine the resistivity of the formation. The resistivity of the formation R at each depth of investigation j is given by:       R    j    =            K      j        ⁢                  V        ringj                    I        sj            
K represents a constant value that is determined by the spacing of the transmitters and receivers.
In addition to using the ring electrodes to determine the resistivity of the formation, a plurality of discrete electrodes also preferably are included to provide images of the borehole. The electrodes may take any of a variety of shapes, including a rectangular or circular shape. The electrodes may mount to the drill string between the toroidal receivers, and preferably are spaced around the circumference of the drilling tool. The voltage of each of the electrodes (Ve) is measured, and used to determine an azimuthally sensitive measure of formation resistivity (Re) for each depth of investigation j.       R    ej    =            K      j        ⁢                  V        ej                    I        sj            
According to the preferred embodiment of the present invention, at least three discrete electrodes are spaced circumferentially around the drill string to provide images of the resistivity at three different orientations. These images may be coordinated with depth and azimuthal orientation to provide a resistivity image of the borehole at certain defined intervals.
In an alternative embodiment, the ring electrodes may be eliminated if a sufficient number of discrete electrodes are provided. In this embodiment, the discrete electrodes are provided around the circumference of the drilling tool, the resistivity values for each of the discrete electrodes may be obtained to determine the azimuthally sensitive formation resistivity as:       R    ej    =            K      j        ⁢                  V        ej                    I        sj            
where Vej is the voltage at each electrode. If the number of button electrodes is sufficient (for example, eight or more discrete electrodes are provided, spaced 45 degrees apart) the ring electrode can be eliminated by summing the voltages of all button electrode to get an apparent ring voltage (Vring):       V    ring    ≅            ∑              i        =        1            N        ⁢          V      ei      
where N represents the number of discrete electrodes and Vei is the voltage at each electrode.
The discrete electrodes comprise a metal structure mounted on the collar. The electrodes are electrically insulated from the collar. In yet another embodiment of the present invention, the electrodes are configured as circular lateral-log arrays, with an inner metal disc surrounded by three outer metal rings. The inner disc and outer rings are all separated by insulating material. These electrodes are operated to force current out of the inner disc by controlling the voltage of the outer rings.
These and other advantages of the present invention will become apparent on reading the detailed description of the invention in conjunction with the drawings.